Your heart is responsible for supplying blood to every living cell in your body. To perform this essential task, your heart must beat in a controlled fashion so it can fill and empty efficiently. Arrhythmias, or abnormal heart rhythms, interfere with your heart’s ability to pump blood and may even become life threatening. Arrhythmias can stem from multiple causes, including heart disease, electrolyte abnormalities, metabolic disorders or toxins. Arrhythmias that occur during sleep could be triggered by low blood oxygen levels due to sleep-disordered breathing or exaggerations of normal sleep-related changes in heart rhythm.
Your nervous and cardiovascular systems undergo profound changes as you shift from an awake to a sleeping state, and further changes occur as you transition from one stage of sleep to another. In some sleep stages, your heart normally slows. In others, such as rapid-eye-movement, or REM, sleep, your heart rate accelerates. According to researchers at Harvard Medical School’s Cardiovascular Institute, these normal fluctuations in rhythm may be amplified in certain individuals -- people with underlying cardiac or respiratory diseases or subtle electrical anomalies in their heart -- leading to excessively slow rhythms, sleep-related atrial fibrillation or other arrhythmias.
Sleep-Related Breathing Disorders
“Sleep-disordered breathing” is a term physicians use to describe conditions that interfere with the normal frequency or depth of breathing during sleep. One of the most common sleep-disordered breathing syndromes is obstructive sleep apnea, or OSA, which is characterized by intermittent episodes of upper airway obstruction during sleep. According to a 2010 review in “Annals of Thoracic Medicine,” arrhythmias are common in people with OSA. Male gender, advancing age and obesity are the major risk factors for OSA. Other sleep-disordered breathing syndromes that may be associated with arrhythmias include central sleep apnea, which is caused by abnormalities in the central nervous system; obesity hypoventilation syndrome, which is characterized by reduced lung movement due to compression of the chest cavity; and upper airway resistance syndrome, which is caused by conditions that reduce airflow through your respiratory passages.
Low Oxygen Is the Trigger
Hypoxia, or low blood oxygen, is the common trigger for arrhythmias in people with sleep-disordered breathing syndromes. As your breathing slows or ceases and your blood oxygen level falls, a number of physiologic events occur that increase your heart’s irritability and set the stage for an arrhythmia. A study published in the June 2009 issue of “Archives of Internal Medicine” listed hypoxia, climbing carbon dioxide levels, stimulation of “fight-or-flight” reflexes and swings in internal chest pressures as potential instigators of nighttime arrhythmias in sleep-disordered breathing syndromes.
A number of studies have shown that sleep-disordered breathing syndromes, particularly obstructive sleep apnea, are associated with a higher risk of mortality. Heart failure and high blood pressure are more common in people with sleep apnea, and a study published in the November 2005 issue of “The New England Journal of Medicine” demonstrated that OSA increases the risk of stroke or death from any cause. Sudden cardiac death during sleeping hours is also more common in people with sleep-related breathing disorders, and a higher incidence of arrhythmias could be the underlying cause.
Sleep-disordered breathing is the most likely cause of a nocturnal arrhythmia, but other factors can alter your heart’s electrical activity during sleep. According to a 2008 review in “Expert Opinion on Drug Safety,” a number of antipsychotic and antidepressant medications have been linked to arrhythmias and even sudden cardiac death. Tricyclic antidepressants, such as imipramine (Tofranil) and nortriptyline (Pamelor), are often taken at bedtime to exploit their sedating side effects, but these drugs are among the most likely to trigger arrhythmias.
Arrhythmias are sometimes discovered during the sleep lab evaluations of people with sleep-disordered breathing syndromes. However, nocturnal arrhythmias may also be uncovered while a person is wearing a 24-hour heart monitor to evaluate irregular heartbeats or palpitations that occur during the day. In the latter case, further investigation with a sleep lab study may identify a previously unsuspected sleep-disordered breathing syndrome. Treatment with nighttime continuous positive airway pressure, or CPAP, often alleviates the arrhythmias, as well as many of the other complications, of sleep-disordered breathing syndromes. Your doctor will determine if your arrhythmia warrants additional evaluation or treatment.